1. Field of the Invention
The invention relates to a unique composite pigment and a method for its preparation, and, more particularly, relates to a composite pigment made by the precipitation of a silicon compound onto precipitated calcium carbonate, and the application of said composite pigment as a filler such as for paper making, a coating pigment, a thickener and/or cleaning agent in toothpaste, a pigment for paint, a conditioner for powders, or as a carrier for liquids or oils.
2. Description of the Related Art
Paper production requires attention to and management of a number of diverse paper quality parameters, such as bulk, opacity, brightness, strength, smoothness, gloss, stiffness, which are important for handability, printability, general appearance and so forth. Fillers are typically used to manipulate one or more of these parameters in a desired manner.
A wide variety of different minerals and synthetic pigments have been investigated and used as fillers in the conventional manufacture of paper. Such paper fillers include, for example, kaolins, calcined clays, titanium dioxide, aluminium trihydrate, ground calcium carbonates, precipitated calcium carbonates, and precipitated silicates. These various types of pigments provide varying overall benefits insofar as the resulting paper qualities and the cost of manufacturing the paper. For example, kaolins are available at low cost, but tend to negatively influence the brightness of the paper compared to the whiter fillers, such as ground marble or PCC. Calcium carbonates, especially PCCs, add scattering power to paper to result in very bright papers, and also permit reductions in production costs. However, it tends to be a challenge to improve other paper properties such as bulk where PCC is the filler, such as by using coarser PCC particles of a given morphology, without adversely impacting and compromising scattering power. Calcined clays and precipitated silicates result in very bright papers with a high opacity and good bulk, but have the disadvantage of being relatively expensive as compared to competing paper fillers.
Consequently, one prior approach has been to combine usage of two or more different types of particulate fillers in paper manufacture in order to combine the respective advantages and/or compensate for the respective drawbacks of the different fillers being added. However, the use of multiple fillers tends to increase the risk of product variability, and also tends to increase material costs and material handling requirements for paper manufacture.
As an alternate approach, the use of composite pigments per se as fillers for paper manufacture has been suggested. Namely, methods have been suggested for manufacturing composite pigment particles constituted by precipitated calcium carbonate and silicon dioxide, and the application of such composite pigments as fillers for the manufacture of paper. For example, WO 95/03251 discloses a process wherein milk of lime (xe2x80x9clime milkxe2x80x9d) and an aqueous solution of sodium silicate is mixed, whereafter a mixed pigment comprising calcium carbonate and silica is precipitated simultaneously by injection of a gas containing carbon dioxide into the mixture until the pH falls to 7 or below. The molar ratio SiO2/CaO is kept at 3.6. The resulting compound pigment is used as filler in paper and purportedly has good optical characteristics. EP 356406 discloses a process wherein a PCC is coated with a zinc silicate in order to make it acid resistant. The zinc silicate is precipitated onto the PCC by a process route wherein a sodium silicate solution and a zinc chloride solution are simultaneously admixed into the PCC suspension and wherein the zinc chloride solution is substituted by a sulphuric acid solution in the latter part of the reaction. GB 1295264 discloses a process wherein calcium silicate is precipitated onto chalk by admixing of a sodium silicate solution into a chalk suspension and stirring it for 30 minutes. The resulting pigment is acid resistant and can be used as filler for xe2x80x9calumxe2x80x9d sized paper. U.S. Pat. No. 5,164,006 discloses a process wherein a silicate is precipitated onto calcium carbonate, e.g. PCC, to prepare an acid resistant calcium carbonate pigment. The U.S. Pat. No. 5,164,006 patent teaches a two stage pH-reduction process for making the pigment wherein the first stage comprises addition of a sodium silicate solution to a calcium carbonate suspension which has a temperature in the range of about 75xc2x0 C. to 80xc2x0 C., and a high amount of excess (unreacted) calcium hydroxide (2-5 g/L excess) and a corresponding high pH (11-12), and reducing the pH to the range of 10.2-10.7 by addition of carbon dioxide gas. Then, in a second stage, the reaction mixture is cooled to about 20xc2x0 C. to 35xc2x0 C. and the pH is further reduced to 7.5-8.0 by adding zinc chloride. Other examples in the description of U.S. Pat. No. 5,164,006 describe a comparative process for making the composite pigment, where the two-stage pH reduction instead is accomplished by addition of carbon dioxide in both stages where the reaction temperature is precipitously lowered from 80xc2x0 C. to 25xc2x0 C. in the middle of the silicon deposition reaction and before performing the second pH-reduction stage of the process.
As generally understood in the field, calcium carbonate may precipitate in three distinct polymorphs: calcite, aragonite and vaterite. The thermodynamically stable product is calcitic PCC. Aragonite and vaterite are both metastable compounds. Normally, vaterite transforms into calcite within a few hours to a day unless special measures have been taken to stabilize the vaterite polymorph. Aragonite, on the other hand, has, for all practical purposes, an infinite shelf life under normal temperatures and pressures. Aragonite precipitates into acicular needles. Calcite on the other hand may precipitate into various crystal habits comprising scalenohedrons and rhombohedrons. In practice, perfect crystal habits are almost only seen in small-scale precipitations performed under well-controlled conditions in the laboratory. Precipitations on a larger scale such as industrial precipitations tend to produce imperfect crystal habits, which may be intermediate in nature, or have defects such as rounded comers. The individual crystals will typically be assembled in aggregates or agglomerates.
Although not related to paper manufacture per se, certain composite calcium carbonate/silicate pigments and methods for manufacture of these also have been proposed for the rubber and polymer industry. Rubber reinforcing agents typically require a high specific surface area (BET). High surface area pigments, namely pigments having surface areas greater than 30 m2/g generally are not suitable for paper manufacturing applications at high loading levels because the high surface areas frustrate the ability to provide a filterable pigment composition for paper manufacturing applications.
GB 838903 discloses a process wherein a calcium silicate is precipitated with a PCC. The silicate solution may be added before, during or after the precipitation of the PCC. However, a starting temperature of 15xc2x0 C. for the carbonation reaction used in the PCC pigment synthesis can be expected to implicitly result in a high surface area PCC product of 20-30 m2/g BET (see FIG. 2 discussion infra). Further deposition of silicates on the PCC particles would only further increase the surface area. Consistently, the background descriptions of GB 838903 indicate that calcium carbonates used for rubber reinforcement are very small, typically 50-100 nm, and that even smaller particles of 30-50 nm are preferred. Such small pigment particles will by nature have relatively high surface areas. U.S. Pat. No. 3,152,001 discloses a process where a fine calcium silicate is precipitated onto a fine PCC by admixing sodium silicate and calcium chloride solutions into the PCC slurry. The products are stated to have specific surface areas above 50 m2/g. U.S. Pat. No. 4,167,423 discloses a process where freshly precipitated metal silicates are admixed with wet ground calcium carbonates. The metal silicates attach to the calcium carbonate in such a way that they can be used as basis for further reaction with organosilane compounds.
The present invention relates to a unique approach for making a composite pigment of precipitated calcium carbonate (PCC) and a silicon compound, in which the resulting composite pigment is endowed with an excellent combination of optical and mechanical properties. Among other things, a PCC/silicate composite pigment made according to this invention imparts improved bulk, light scattering power, porosity, roughness, and printing properties to paper. The composite pigment material made according to this invention also is very competitive from the standpoint of its production cost.
As a general embodiment, the present invention relates to a method for making a composite pigment wherein a soluble silicate compound is introduced into an aqueous medium containing a precipitate of calcium carbonate formed by carbonation of lime milk, and at a time when the calcium carbonate precipitation reaction has progressed to near completion. Then, an insoluble silicon compound is precipitated upon the precipitated calcium carbonate by carbonation of the reaction mixture in a manner in which the maximum temperature variation of the reaction mixture is kept less than 20xc2x0 C. In one preferred embodiment, the soluble silicate compound is added when the precipitation of the calcium carbonate has reached approximately 90% to less than 100% of completion, as calculated as a ratio of the original molar amount of calcium supplied via the calcium hydroxide reactant that has been consumed so far to form the calcium carbonate intermediate product divided by the original molar amount of calcium. In another preferred embodiment, the temperature of the reaction mixture during deposition of the silicon compound upon the precipitated calcium carbonate is at least 50xc2x0 C. or greater, and more preferably is kept between approximately 60 to 100xc2x0 C., and more preferably between 70 to 90xc2x0 C.
By controlling the precipitation reaction conditions according to the above-mentioned protocols, the inventive method is conducted within a unique processing envelope so as to achieve a composite precipitated calcium carbonate/silicon compound pigment having a special morphology and constitution, which is well-suited and versatile for use as a paper filler, a paint pigment, and the like. For paper filler or pigment applications, the composite pigments of this invention can be provided in aqueous slurry form or dry form. The aqueous slurry forms of the inventive composite pigments also optionally can include various other kinds of additives conventionally used in compositions for papermaking. Also, if dried, the inventive pigment solids can be used in a dry, flowable particulate form for incorporation into a dentifrice as a thickener or cleaning agent, or as a carrier for oils and liquids, and the like.
Other aspects and preferred embodiments will be apparent from the following detailed description of the invention.